26 U.S.C. § 4072

1. (a)For purposes of this chapter, the term “taxable tire” means any tire of the type used on highway vehicles if wholly or in part made of rubber and if marked pursuant to Federal regulations for highway use.
2. (b)For purposes of this chapter, the term “rubber” includes synthetic and substitute rubber.
3. (c)For purposes of this part, the term “tires of the type used on highway vehicles” means tires of the type used on—Such term shall not include tires of a type used exclusively on vehicles described in section 4053(8).
   1. (1)motor vehicles which are highway vehicles, or
   2. (2)vehicles of the type used in connection with motor vehicles which are highway vehicles.
4. (d)For purposes of this part, the term “biasply tire” means a pneumatic tire on which the ply cords that extend to the beads are laid at alternate angles substantially less than 90 degrees to the centerline of the tread.
5. (e)For purposes of this part, the term “super single tire” means a single tire greater than 13 inches in cross section width designed to replace 2 tires in a dual fitment. Such term shall not include any tire designed for steering.